Electric current-limiting low voltage fuse

ABSTRACT

An electric current-limiting low voltage fuse for back-up protection of switching devices having relatively movable contacts and being controlled by excess current tripping devices includes low resistance fusible elements in ribbon form and a high resistance fusible element in form of a helically wound wire. On occurrence of major fault currents the low resistance and the high resistance fusible elements fuse and vaporize sequentially. Fusion and vaporization of the low resistance fusible elements limits the peak let-through current to a value way below the peak of the available short-circuit current. The relatively small current that flows through the high resistance fusible element affords sufficient time for the switching device to open the faulted circuit completely.

United States Patent 1191 Kussy et al.

[ Oct.8, 1974 ELECTRIC CURRENT-LIMITING LOW VOLTAGE FUSE "116163615:rraarwiias Havrford', Pal

Filed: Sept. 4, 1973 Appl. No.: 394,290

US. Cl 337/159, 337 162, 337/164, 337 293 Int. Cl. H01h 85/04 Field OfSearch 337/159, 161, 162, 163, 1 337/164, 293

[56] References Cited UNITED STATES PATENTS 2,216,661 10/1940 Dannenberg337/293 X 2,249,945 7/1941 Dannenberg 337/293 X Thfihase ShawmutCompany, N

Primary Examiner-J. D. Miller Assistant Examiner-Fred E. Bell Attorney,Agent, or Firm-Erwin Salzer 57 ABSTRACT An electric current-limiting lowvoltage fuse for backup protection of switching devices havingrelatively movable contacts and being controlled by excess currenttripping devices includes low resistance fusible elements in ribbon formand a high resistance fusible element in form of a helically wound wire.On occurrence of major fault currents the low resistance and the highresistance fusible elements fuse and vaporize sequentially. Fusion andvaporization of the low resistance fusible elements limits the peaklet-through current to a'value way below the peak of the availableshort-circuit current. The relatively small current that flows throughthe high resistance fusible element affords sufficient time fortheswitching device to open the faulted circuit completely.

FIG.4.

PATENTED 974 SHEEI 2 BF 2 time I ELECTRIC CURRENT-LIMITING LOW VOLTAGEFUSE " BACKGROUND OF THE INVENTION In view of the continuous increase ofthe magnitude of available major fault currents in industrial and othersystems it is necessary, to an ever increasing extent, to protectelectrical apparatus, e.g. circuit breakers, by means ofcurrent-limiting backup fuses.

It is possible to coordinate circuit breakers or other motor startingequipment and current-limiting back-up fuses for the latter eithermechanicallyor electrically.

In the first'mentioned instance blowing of the fuse retime of thecircuit breaker may be too long to cause tripping of the circuit breakerby the let-through current of its back-up fuse, or the desiredcurrent-limiting action of the back-up fuses could not be achieved ifthey were selected with the object in mind of causing tripping of agiven circuit breaker by the let-through current thereof.

The present invention resolves these conflicts.

SUMMARY OF THE INVENTION Fuses embodying this invention include inaddition to the current-limiting fusible elements a helically woundfusible element of resistance wire which fuses at a later point of timethan the current-limiting fusible element, thus extending the durationof current flow to cause tripping of a relatively slow automaticswitching device or circuit breaker. Both the current-limiting fusibleelements and the helically wound resistance wire are submersed in thesame or a unitary body of pulverulent arc-quenching filler. Thehelically wound fusible element is arranged close to a median plane ofthe fuse tube inside of a space bounded by the current-limiting fusibleelements and sandwiched between the currentlimiting fusible elementsline 17, which form parallel current paths arranged symmetrically inregard to said median plane.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an exploded isometric viewof a fuse and fuse holder combination embodying this invention and alsoshowing a portion of a molded case circuit breaker intended to be backedup on occurrence of major fault currents;

FIG. 2 is a diagrammatic top-plan view of a fuse holder as shown in FIG.1 without any fuses therein;

FIG. 3 is a top-plan view of a fuse embodying this invention;

FIG. 4 is substantially a longitudinal section of the fuse of FIG. 3,and

FIG. 5 are traces illustrating the operation of the composite circuitbreaker and fuse unit shown in FIG. 1.

DESCRIPTION OF PREFERRED EMBODIMENT Referring now to the drawings,numeral 1 has been applied to designate a tubular casing or fuse tube ofa current-limiting fuse. Fuse tube 1 is of an electric insulatingmaterial, e.g. melamine glass cloth. The ends of easing l are closed bya pair of plug terminals 2, which are press-fitted into casing 1. Steelpins 3a, 3b, 3c,3d

TRIC FUSE HAVING STRESS-REDUCING FUSE LINK MEANS. Each fusible elementor fuse link 4 is sandwiched between a pair of plates 4a of insulatingmaterial for controlling the arc-voltage generated incident to blowingof the fuse, as more fully explained in US. Pat. No. 2,964,604 to P. C.Jacobs, Jr. et 211:. Dec. 13, 1960 for CURRENT LIMITING FUSES HAVINGCOMPOUND ARC-VOLTAGE MEANS. The axially inner surfaces of plug terminals2 are provided with grooves engaged by the axially outer ends of fuselinks 4. The aforementioned grooves are filled with soft soldertoconductively connect fuse links 4 and plug terminals 2. Fuse links 4are shunted by a high resistance restraining'wire 6 for a blown fuseindicator of the kind more fully disclosed in US. Pat. No. 3,391,369 toF. .I. Kozacka, July 2, 1968 for HIGH VOLTAGE FUSE. Fuse links 4 arefurther shunted by a helically wound resistance wire 5 of Nichrome. Theends of the latter are inserted into recesses or bores in plug terminals2 and conductively connected to the latter by masses of a low fusingpoint metal. This may be. 1

a tin-free solder. It is not possible to form a solder joint between themetal of which plugs 2 are made and the ends of Nichrome wire 5.However, the masses of low fusing point metal surrounding the ends ofNichrome wire 5 exert such friction on wire 5 as to firmly 'position theends thereof inside the bores or recesses into which they are inserted.This resistor 5 is our joint invention, but the fuse holder describedbelow is not our joint invention. It is claimed in a copending patentapplication of Bernard DiMarco et al., filed Jan. 16, 1973, Ser. No.324,193 for FUSE HOLDER WITH REJECTION FEATURE. The upper plug terminal2 is provided with a wire connector generally indicated by referencenumeral 7 and including the set-screw 8. The lower plug terminal 2 isprovided with a substantially Z-shaped terminal strap 9 i.e., aconnector strap similar in shape to an inverted Z affixed to it byscrews 10. Connector straps 9 include web-portions extending parallel tothe axis of easing 1 and pairs of flange portions extending at angles ofin opposite directions away from the web portions. The upper flangeportion of each terminal strap 9 forms a lug intended to engage acorresponding terminal in the molded case circuit breaker 11 shown inFIG. 1. Reference numeral 12 has been applied in FIG. 1 to indicate ascrew intended to screw the upper flange portion of terminal strap 9against a cooperating contact surface forming part of the molded casecircuit breaker 11. The molded case circuit breaker 11 is provided witha fuse holder or pouch generally indicated by reference numeral 13. Fuseholder or pouch GENERATING v 13 is a molding of a plastic or syntheticresin forming three substantially tubular receptacles 13a for insertionof a fuse into each of them. Each receptacle 13a may be provided withone or more internal groove means extending in a directionlongitudinally thereof. One or more of the fastener pins 3a-3d forfastening the lower plug terminal 2 (FIGS. 3 and 4) to casing 1 projectradially outwardly beyond the cylindrical surface of casing 1. Thereforea receptacle 130 can only receive a fuse conforming to its shape andsize if thereis a groove 14 juxtaposed to pins 3a3d projecting beyondthe general outline of the fuse. It is possible to vary the number ofprojecting pins and the position thereof, and to provide fuse holders 13which have grooves 14a whose number and position correspond to thenumber and position of the pins of the particular fuse that is intendedto be accepted by the fuse holder.

to minimize the interrupting duty of each. The interrupting duty of fuseF is much more severe than that of a conventional back-up fuse becausethe presence of resistor 5 results in a drastic increase of metal vaporincident to blowing of the fuse and because that metal vapor has arelatively low ionizing potential. Considering relatively high currentratings of fuse F, the positioning of the helically wound resistors 5relative to the other parts of the fuse becomes a factor of considerablesignificance. To be more specific, in such instances the resistor 5ought to be arranged as shown in FIG. 4, i.e., resistor 5 ought to bearranged relatively close to a median plane of the fuse tube 1 and thefusible elements The top-plan view of the fuse holder 13 shown in FIG. 2is diagrammatic. The fuse holder forms 3 receptacles 13a closed at thebottom and open at the top. Each receptacle 13a is closed at the frontthereof and is provided with a slot on the rear thereof allowing theupper flange portion of terminal straps 9 to project out of receptacles13a into engagement with corresponding terminals of molded case circuitbreaker l1. Slots l3 and straps 9 determine the position of each of thethree fuses relative tothe fuse holder 13, and thus a fuse can only beinserted into the fuse holder if both have corresponding projecting pins3a-3d and pinengaging grooves l 4.

In FIG. 2 the possible positions of four grooves 14a have been indicatedand numbered 1 ',2,3,4'. Considering a fuse holder 13 with a singlegroove having the position 1 and a fuse having one single projecting pin3a, pin 3a cannot engage the groove positioned at '1' because the fuseis positioned relative to fuse holder 13 by straps 9 of the former andthe slots 15 of the latter. A fuse having a single projecting pin 3apositioned as shown in FIG. 3 can only be inserted into a fuse holderhaving groove means positioned at.4'..

The resistor 5 of FIG. 4 makes it possible to achieve electricalcoordination of a circuit breaker and of its back-up current limitingfuses even in instances where the circuit breaker is relatively slow andwhere the letthrough i -t value of the fuses is relatively small.

FIG. 5 explains more in detail the operation of shunt resistor 5. FIG. 5is a plot of current versus time, i being the trace of an availableshort-circuit current i being the trace of the actual short-circuitcurrent or, in other words, that of the available short-circuit currentas modified by the operation of the fuse. The actual shortcircuitcurrent i rises initially at the same rate as the availableshort-circuit current i and the former decays rapidly following meltingand vaporization of the fusible elements 4. After reaching an initialcurrent zero the fault current limited by resistor 5 continues to flowfor a period of time sufficient to cause tripping of the molded casecircuit breaker 11.

For fuses embodying this invention the problem of 4 ought to be arrangedrelatively remote from said median plane sandwiching resistor winding 5.As a result of this configuration a zone of relatively high and uniformtemperature is formed adjacent the center region of resistor winding 5.This results in a tendency of formation of a very long initial break, orof series breaks,

at the aforementioned zone of relatively high and uniform temperature.

It may happen incident to some fault condition that fusible elements 4fuse and are vaporized by resulting arcing, that the letthrough currentcauses'opening of the backed-up circuitbreaker or other motor startingdevice, but that the resistor 5 remains intact, i.e. is not melted andvaporized. Reclosing of the circuit breaker or other motor startingdevice then causes reenergization of resistor 5. Under such conditionsit'is necessary to effect rapid interruption of the current flow throughresistor 5 by melting and vaporization of the latter.

It may be pointed out that a molded case circuit breaker or other motorstarting device and a back-up fuse may be designed from the very outsetto form a matched pair. In that case the problem underlying thisinvention does not arise.

We claim as our invention:

1. An electric current-limiting low voltage fuse for backup protectionof switching devices having relatively movable contacts and beingcontrolled by excess current tripping devices, said fuse including incombination a. a fuse tube of electric insulating material;

b. a pair of terminal elements closing the axially outer ends of saidfuse tube;

c. a unitary body of pulverulent arc-quenching filler inside said fusetube;

d. low resistance current-limiting fusible elements in ribbon formconductively interconnecting said pair of terminal elements submersed insaid unitary body of filler and being arranged symmetrically to oppositesides of a median plane of said fuse tube to form separate current pathssituated to opposite sides of said median plane and relatively remotefrom said median plane; and

e. a high resistance fusible element in form of a helically wound wireconductively interconnecting said pair of terminal elements, submersedin said unitary body of filler and being arranged relatively close tosaid median plane of said fuse tube inside of a space bounded by saidlow resistance currentlimiting fusible elements so as to be sandwichedbetween said low resistance fusible elements. 2. A fuse as specified inclaim 1 wherein said helically wound wire is of Nichrome.

3. An electric current-limiting low voltage fuse for back-up protectionof switching devices having relatively movable contacts and beingcontrolled by excess current tripping devices, said fuse including incombination being arranged relatively remote from a median plane of saidfuse tube; and

e. a high resistance fusible element in form of a helically woundNichrome wire conductively interconnecting said pair of plugs submersedin said filler andbeing arranged relatively close to said median planeof said fuse tube and being sandwiched between said low resistancefusible elements, said pair of juxtaposed bores in saidaxially inner endsurfaces of said pair of plugs receiving theaxially outer ends of saidhelically wound wire of Nichrome, and said bores further containing amass of low fusing point metal frictionally positioning said ends.

1. An electric current-limiting low voltage fuse for backup protectionof switching devices having relatively movable contacts and beingcontrolled by excess current tripping devices, said fuse including incombination a. a fuse tube of electric insulating material; b. a pair ofterminal elements closing the axially outer ends of said fuse tube; c. aunitary body of pulverulent arc-quenching filler inside said fuse tube;d. low resistance current-limiting fusible elements in ribbon formconductively interconnecting said pair of terminal elements submersed insaid unitary body of filler and being arranged symmetrically to oppositesides of a median plane of said fuse tube to form separate current pathssituated to opposite sides of said median plane and relatively remotefrom said median plane; and e. a high resistance fusible element in formof a helically wound wire conductively interconnecting said pair ofterminal elements, submersed in said unitary body of filler and beingarranged relatively close to said median plane of said fuse tube insideof a space bounded by said low resistance currentlimiting fusibleelements so as to be sandwiched between said low resistance fusibleelements.
 2. A fuse as specified in claim 1 wherein said helically woundwire is of Nichrome.
 3. An electric current-limiting low voltage fusefor back-up protection of switching devices having relatively movablecontacts and being controlled by excess current tripping devices, saidfuse including in combination a. a fuse tube of electric insulatingmaterial; b. a pair of terminal elements in the form of a pair of plugsplugging the axially outer ends of said fuse tube, the axially inner endsurfaces of said pair of plugs being provided with a pair of juxtaposedbores; c. a pulverulent arc-quenching filler inside said fuse tube; d.low resistance current-limiting fusible elements in ribbon formconductively interconnecting said pair of terminal elements, submersedin said filler and being arranged relatively remote from a median planeof said fuse tube; and e. a high resistance fusible element in foRm of ahelically wound Nichrome wire conductively interconnecting said pair ofplugs submersed in said filler and being arranged relatively close tosaid median plane of said fuse tube and being sandwiched between saidlow resistance fusible elements, said pair of juxtaposed bores in saidaxially inner end surfaces of said pair of plugs receiving the axiallyouter ends of said helically wound wire of Nichrome, and said boresfurther containing a mass of low fusing point metal frictionallypositioning said ends.